In many communication system architectures, such as in the context of digital video broadcast (DVB), the same content is broadcast on various radio channels from one or several transmitters. In radio communication systems a received signal can be switched to a different channel if a reception quality indicator or level drops below a certain threshold. This procedure is known as a handover.
The radio channel may vary over time, because a receiver is moving within the communication network or there is, for instance, a moving object passing the receiver. Therefore, the receiver may have to measure the quality of different radio channels to decide which channel offers sufficient quality of service and to perform a handover to that channel.
For time division multiplexed signals, the receiver can try to receive another frequency when no signal is transmitted on its current reception frequency.
The power consumption of such a receiver is an important issue. The receiver may be a battery powered device, such as a cellular phone or any type of mobile communication device. Therefore, the receiver has to evaluate a reception quality indicator for the current radio channel and the potential handover channel as fast as possible.
Different reception quality indicators exist that indicate a need for a handover. One such an indicator is the received radio frequency power or signal strength. The receiver makes a decision whether to perform a handover after having evaluated received signal strengths from different channels. The evaluation can be based on fixed thresholds or comparisons of different channels. Other examples of different reception quality indicators are: fixed received pilot signal power, relative received pilot signal power or received signal-to-noise ratio.
The major drawback when these indicators are employed is that the handover decisions are not very reliable for large band signals in multi path or fading channel environments. These channels experience frequency selective and time varying fading which alter the meaning of an absolute power for received pilot signals. Even a relative threshold between the different received channels cannot reliably prove that the receiver is able to demodulate the corresponding signal.
A publication entitled “Viterbi decoder synchronization and BER measurement using metric values” by Richard W. Koralek et al shows that the distribution of metric values in a Viterbi decoder depends on the Eb/No of the communication link. A particular measure of the metric distribution is developed for synchronization and error rate purposes.
European patent application publication 0 865 176 A2 relates to a receiver of transmission wave in which a specific information data signal and its validity identification data signal are transmitted over the same frequency band.